Abstract:

Background and Aims: Intensive fieldwork along the shore of the East Cape Region in the Baja California Peninsula by the authors has resulted in the discovery and publication of some new species for the Mexican flora in recent years.

Methods: An additional novelty found was a tree considered at first to be Bursera microphylla, but a more detailed inspection showed the leaflets to be few in number and broadly obovate-elliptic. These and additional differences of the leaves and flowers warrant recognition of this plant as a new species.

Key results: We here describe and illustrate Bursera exequielii (Burseraceae), which belongs to section Bursera and we discuss its morphological relationship with B. microphylla, from which it differs mainly by the number, form and size of the leaflets.

Conclusions: The only known population is threatened due to the mega-touristic developments in the zone.

Introduction

Bursera is an American genus with around 100 species, most of them native to the Mexican dry tropics on the Pacific slope (Espinosa et. al., 2006; De Nova et al., 2012). According to Rzedowski (2015) the current number of Bursera taxa in Mexico is 89. It is a difficult taxonomic group because it is dioecious and mostly deciduous, and the leaves are often absent from flowering or fruiting specimens. As a consequence, field observations and several collections are necessary. Moreover, vegetative structures are difficult to preserve and analyze in voucher specimens since they easily disassemble at drying. Hence, Bursera taxonomy is based on morphological characteristics of leaves, flowers, fruits, and bark when observed mainly in fresh material, at the time of collecting (Rzedowski et al., 2004).

Material and methods

As a result of intensive fieldwork along the Baja California Peninsula during a project to document the floristic composition of the beaches of Mexico (Espejel Carbajal et al., 2015), the eastern shore of the southern Baja California Sur, known as the East Cape Region, was visited by the authors in different locations for plant collecting and vegetation sampling, resulting in the discovery of some interesting plants (León de la Luz and Levin, 2012; León de la Luz and Medel Narváez, 2013). An additional novelty found was a tree located just beside a beach, on a sandy marine terrace. This tree was considered at first to be a “torote rojo”, Bursera microphylla A. Gray, one of the most common sarcocaulescent trees along most of the Baja California peninsula from sea level up to 1000 m, and whose natural distribution “almost is coincident with the Sonoran Desert province” (Turner et al., 1995). However, a more detailed inspection showed the leaflets to be few in number and broadly obovate-elliptic. These and additional differences of the leaves and flowers warrant recognition of this plant as a new species.

Observations

The exfoliating bark, winged leaf rachis, and 3-valved fruits place this species in section Bursera, the “cuajiote” group in the Mexican mainland vernacular nomenclature (McVaugh and Rzedowski, 1965; Rzedowski et al., 2004), also known by the bajacalifornians as “torote”.

In addition to belonging to section Bursera, B. exequielii and B. microphylla both have tortuous branching and similar flower and fruit morphology, but the new species differs from B. microphylla by the 7-11 (vs. 7-35) leaflets that measure 7-9 × 5-6 mm (vs. 5-25 × 1-3) and are broadly elliptic to obovate (vs. linear-oblong to elliptic or lanceolate). Moreover, inflorescences are denser in B. microphylla.

Hence, B. exequielii seems to be a local population possibly descendent from the widely distributed B. microphylla. Following the proposal of Becerra (2003), who reconstructed a phylogenetic hypothesis of 66 species and varieties of Bursera using parsimony analyses, this new species must be included in the section Bursera and Microphylla group. In addition, De-Nova et al. (2012) published a Bursera phylogeny at species level by estimating divergence times, temporal diversification heterogeneity, and geographical structure, in order to determine when species could have first appeared in Mexican territory. With respect to B. microphylla they assessed that this species established some 8 My ago; i.e., just when the peninsular territory split off from the Mexican mainland by the opening of the Gulf of California (Ferrari et al., 2007).

Several botanists have noticed that across the broad range of B. microphylla¸ it exhibits variation in leaf and leaflet size, but not in form. Johnson (1992) illustrated such differences for several populations of Sonora, Mexico (see Fig. 3), which show significant variation in leaflet size but a consistent length/width ratio ≥4, and the leaflets are consistently linear-oblong to narrowly lanceolate. Flower sex of B. microphylla has been barely documented. Available bibliography, such as Shreve and Wiggins (1964), relates the species as dioecious, whereas Felger (2000) stated that in B. microphylla sepals and petals are usually 3 on female flowers and (4)5 on male flowers. In B. exequielii we observed that female flowers have 3(4) sepals and petals, and male flowers (4)5(6) sepals and petals. Moreover, few fruits were seen on male individuals, but due to its low frequency this trait could be considered rather as unusual functional pistils in staminate flowers more than genuine hermaphrodite flowers that would give the polygamodioecious character to the species. The number of cymes per cluster is also a noticeable difference, since in B. exequielii there are commonly 2-3 cymes, whereas in B. microphylla there are 4-6, although both taxa could have 2-3 flowers per cyme.

Fruit morphology of the two species is similar in shape and size, but the pseudoaril of B. microphylla seems to be more reddish than yellowish. An additional difference could be noticed in the sprouting of the flowers: in B. microphylla leaves and flowers are produced synchronously (Johnson, 1992; see Fig. 2F), while in B. exequielii we observed first the flower development (Fig. 2D, E), and the bud leaves appear a couple of weeks later; both structures continue growing during the rainy season.

Ecology

The main population of this taxon inhabits a marine terrace of Holocene origin (DEGETENAL, 1981), formed by sand deposited by the sea surf. Bursera exequielii seems to be the only species of Bursera or “torote” in this zone where it grows with spinescent shrubs such as Ebenopsis confinis (Stand.) Britt. & Rose, Sideroxylonoccidentale (Hems.) T.D. Penn., Condalia globosa I.M. Johnston, Paulothamnus spinescens A. Gray, sarcocaulescent forms as Jatropha cinerea (Ortega) Müll.-Arg., Forchhammeria watsonii Rose, Cyrtocarpa edulis (Brandegee) Stand., and cacti such Cylindropuntia cholla (F.A.C. Weber) F.M. Knuth, C. alcahes (F.A.C. Weber) F.M. Knuth, and Stenocereus gummosus (Engelm.) A. Gibson & K.E. Horak. We estimated the density of plants of this new species at 15 to 20 per hectare. The individuals appear vigorous, but we saw no evidence of regeneration. Some individuals were observed up to 5 km inland, mixed in the scrubland with B. microphylla on calcareous soils. Intermediate forms were not found.

Etymology

The taxon is named in honor of Dr. Exequiel Ezcurra (University of California, Riverside), an ecologist and leading conservationist who has tirelessly researched and defended the peninsula of Baja California.

Authors contributions

RDC worked on the herbarium vouchers and realized the specimen was possibly a new taxon. JLLL and AMN visited the population to collect more samples to verify this. JLLL, RDC and AMN discussed the possibility of these samples belonging to a new taxon. RDC and AMN carried out the morphological observations. JLLL wrote the manuscript in collaboration with RDC and AMN. All authors contributed to the description, discussion and conclusions.

Funding

Acknowledgements

We express our gratitude to several persons that helped in several stages of this work. We are grateful to our elder colleague Miguel Domínguez L. for field assistance, Victor Steinmann for encouraging the study of this taxon, and Rosalinda Medina Lemos for reviewing an earlier version of the manuscript. The authors are also grateful to Danira León C. for an earlier drawing, to Albino Luna for the illustration of the species which is published here, to Fernando Chiang Cabrera for the Latin diagnosis, and to the anonymous referees of Acta Botanica Mexicana whose observations and comments improved the final version of the manuscript.

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